1. Field of the Invention
The present invention relates to a private key delivery system and a private key delivery method.
2. Description of the Related Art
Safe and secure cryptocommunications are indispensable for realization of a next-generation information society where such as electronic government and electronic commerce are conceived. In crypto-communications, a public key method and a private key method are used. One practice of the public key method includes an RSA public key method that is widely used at present, information security of which is based only on the principle that a long time is required to acquire factorization of a very large number into prime factors by a polynomial, that is, it takes a long time to decode, there being a huge amount of computation required. Therefore, if a quantum-improvement computer capable of performing high-speed parallel computing becomes available, the time required of decoding a long code such as RSA code will be shortened by leaps and bounds. Then, if a public key in the public key method can be decoded by high-speed computing, there could be tapping of data and fear of alteration by a third party, degrading information security to a level far from perfect. Further, according to the private key method, a sender and a receiver share the same private key, the sender enciphers data by the private key, and the receiver decodes the enciphered data with the private key. Further, in the private key method, there is a possibility of the private key being intercepted by a third party when the private key is distributed to the sender and the receiver. That is, information security is not perfect.
As means to solve the problem relating to information security as described above, use of quantum coding is considered to be promising. As for quantum coding, various methods are proposed, e.g., BB84 (proposed by C. H. Bennett and G. Brassard in 1984), and E91 (proposed by A. K. Ekert in 1991). In the case of BB84, for example, information is carried and transmitted by each photon instead of an aggregate of photons like conventional optical communication. For example, if one bit of information is represented by one of polarization states of a photon, the bit cannot be copied or taken out from the photon without destroying the state of the photon. This is because the behavior of the photon follows Heisenberg's uncertainty principle (the principle that physical quantities of a conjugate cannot be correctly measured simultaneously), and the no-cloning theorem (the theorem that reproduction of a quantum state cannot be copied without observing the quantum state). Therefore, although tapping a communication path by a third party cannot be prevented in the quantum coding method, tapping (copying and alteration) can be detected. In this way, where an information bearer is a single photon, the information security of the private key shared by the sender and the receiver is guaranteed based on a physical principle, not the computational complexity/difficulty.
In recent years and continuing, quantum code systems are being put into practical use. A quantum code system includes a transmitting side that includes a single photon generating unit for generating a photon (single photon), and a polarization state control unit for assigning information of a private key to a photon; and a receiving side that includes a single photon detecting unit for detecting the information on the photon. As the single photon generating unit, a laser light source and an attenuator are usually used, wherein a laser pulse train is irradiated from a laser light source, luminous intensity of which is attenuated by the attenuator such that the average number of photons per pulse becomes 1 or less. That is, the single photon generating unit generates a pseudo-single photon.
[Patent reference 1] JPA 2003-249928
[Patent reference 2] JPA 2000-216775